Network media is the actual path over which an electrical signal
travels as it moves from one component to another. This chapter describes the
common types of network media, including twisted-pair cable, coaxial cable,
fiber-optic cable, and wireless.

Twisted-Pair Cable

Twisted-pair cable is a type of cabling that is used for telephone
communications and most modern Ethernet networks. A pair of wires forms a
circuit that can transmit data. The pairs are twisted to provide protection
against crosstalk, the noise generated by adjacent pairs. When electrical
current flows through a wire, it creates a small, circular magnetic field around
the wire. When two wires in an electrical circuit are placed close together,
their magnetic fields are the exact opposite of each other. Thus, the two
magnetic fields cancel each other out. They also cancel out any outside magnetic
fields. Twisting the wires can enhance this cancellation effect. Using
cancellation together with twisting the wires, cable designers can effectively
provide self-shielding for wire pairs within the network media.

UTP Cable

UTP cable is a medium that is composed of pairs of wires (see Figure
8-1).UTP cable is used in a variety of networks. Each of the eight
individual copper wires in UTP cable \is covered by an insulating material.
In addition, the wires in each pair are twisted around each other.

UTP cable relies solely on the cancellation effect produced by the twisted
wire pairs to limit signal degradation caused by electromagnetic interference
(EMI) and radio frequency interference (RFI). To further reduce crosstalk
between the pairs in UTP cable, the number of twists in the wire pairs varies.
UTP cable must follow precise specifications governing how many twists or braids
are permitted per meter (3.28 feet) of cable.

UTP cable often is installed using a Registered Jack 45 (RJ-45) connector (see
Figure 8-2). The
RJ-45 is an eight-wire connector used commonly to connect computers onto a local-area
network (LAN), especially Ethernets.

When used as a networking medium, UTP cable has four pairs of either 22- or
24-gauge copper wire. UTP used as a networking medium has an impedance of 100
ohms; this differentiates it from other types of twisted-pair wiring such as
that used for telephone wiring, which has impedance of 600 ohms.

UTP cable offers many advantages. Because UTP has an external diameter of
approximately 0.43 cm (0.17 inches), its small size can be advantageous during
installation. Because it has such a small external diameter, UTP does not fill
up wiring ducts as rapidly as other types of cable. This can be an extremely
important factor to consider, particularly when installing a network in an older
building. UTP cable is easy to install and is less expensive than other types of
networking media. In fact, UTP costs less per meter than any other type of LAN
cabling. And because UTP can be used with most of the major networking
architectures, it continues to grow in popularity.

Disadvantages also are involved in using twisted-pair cabling, however. UTP
cable is more prone to electrical noise and interference than other types of
networking media, and the distance between signal boosts is shorter for UTP than
it is for coaxial and fiber-optic cables.

Although UTP was once considered to be slower at transmitting data than other
types of cable, this is no longer true. In fact, UTP is considered the fastest
copper-based medium today. The following summarizes the features of UTP
cable:

Speed and throughput10 to 1000 Mbps

Average cost per nodeLeast expensive

Media and connector sizeSmall

Maximum cable length100 m (short)

Commonly used types of UTP cabling are as follows:

Category 1Used for telephone communications. Not suitable
for transmitting data.

Category 2Capable of transmitting data at speeds up to 4
megabits per second (Mbps).

Category 3Used in 10BASE-T networks. Can transmit data at
speeds up to 10 Mbps.

Category 4Used in Token Ring networks. Can transmit data at
speeds up to 16 Mbps.

Category 5Can transmit data at speeds up to 100
Mbps.

Category 5e Used in networks running at speeds up to 1000
Mbps (1 gigabit per second [Gbps]).

Shielded Twisted-Pair Cable

Shielded twisted-pair (STP) cable combines the techniques of shielding,
cancellation, and wire twisting. Each pair of wires is wrapped in a metallic
foil (see Figure 8-3).
The four pairs of wires then are wrapped in an overall metallic braid or foil,
usually 150-ohm cable. As specified for use in Ethernet network installations,
STP reduces electrical noise both within the cable (pair-to-pair coupling, or
crosstalk) and from outside the cable (EMI and RFI). STP usually is installed
with STP data connector, which is created especially for the STP cable. However,
STP cabling also can use the same RJ connectors that UTP uses.

Although STP prevents interference better than UTP, it is more expensive and
difficult to install. In addition, the metallic shielding must be grounded at
both ends. If it is improperly grounded, the shield acts like an antenna and
picks up unwanted signals. Because of its cost and difficulty with termination,
STP is rarely used in Ethernet networks. STP is primarily used in Europe.

The following summarizes the features of STP cable:

Speed and throughput10 to 100 Mbps

Average cost per nodeModerately expensive

Media and connector sizeMedium to large

Maximum cable length100 m (short)

When comparing UTP and STP, keep the following points in mind:

The speed of both types of cable is usually satisfactory for local-area
distances.

These are the least-expensive media for data communication. UTP is less
expensive than STP.

Because most buildings are already wired with UTP, many transmission
standards are adapted to use it, to avoid costly rewiring with an alternative
cable type.